The present invention relates to a method for transmitting feedback information for beam-shaping in a radio communication system having a number of subscriber stations as well as a base station, whose associated antenna device has several antenna elements so that spatial resolution is possible during beam-shaping. The present invention further relates to a subscriber station suitable for use with such method.
In radio communication systems, messages, (voice, video information or other data) are transmitted by transmission channels with the aid of electromagnetic waves (a radio interface). The transmission takes place both in the downwards direction (downlink) from the base station to the subscriber station and in the upwards direction (uplink) from the subscriber station to the base station.
Signals that are transmitted using electromagnetic waves are subject to interference during their propagation in a propagation medium, mainly by disturbances. Disturbances due to noise mainly occur due to noise in the input stage of the receiver. Signal components pass through different propagation paths due to diffraction and reflection. Consequently, a signal can arrive at a receiver several times, from different directions, with different delays, attenuations and phase angles in each case. Moreover, parts of the reception signal can coherently superimpose at the receiver with changing phase relationships and lead to fast fading there.
From DE 197 12 549 A1, the use of smart antennas is known (i.e., antenna arrangements with several antenna elements), to increase the transmission capacity in the upwards direction. This enables the deliberate alignment of the antenna gain in a direction from which the uplink signal comes.
From A. J. Paulraj, C. B. Papadias, “Space-time processing for wireless communications”, IEEE Signal Processing Magazine, November 1997, p. 49–83, various methods are known for the spatial separation of uplink and downlink signals.
For the downlink direction (i.e., from the base station to the subscriber station), particular difficulties occur because the beam-shaping has to be performed before the influencing of the transmitted signals through the radio channel. From R. Schmalenberger, J. J. Blanz, “A comparison of two different algorithms for multi antenna C/I balancing”, Proc. 2nd European Personal Mobile Communications Conference (EPMCC), Bonn, Germany, September 1997, p. 438–490, an algorithm for beam-shaping in the downwards direction is known, whereby a direct propagation path (line of sight link) between the base stations and the subscriber stations and an iterative calculation of beam-shaping vectors is presumed. The expensive iterative calculation must be completely repeated each time the properties of the transmission channel change.
From DE 198 03 188 A, a method is known whereby a spatial covariance matrix is determined for a connection from a base station to a subscriber station. At the base station, a characteristic vector is calculated for the covariance matrix and used for the connection as a beam-shaping vector. The transmitted signal for the connection is weighted with the beam-shaping vector, and antenna elements are applied for the radiation. Intracell interference is not included in the beam-shaping because of the use of joint detection, such as in the terminals, and any corruption of the received signals due to intercell interference is ignored.
Clearly, in an environment with multipath propagation, this method produces a propagation path with good transmission properties and concentrates the transmission power of the base station spatially on this propagation path. However, this does not mean that interference on this transmission path could not lead to fast fading of the signal and, thus, to interruptions in the transmission.
The recommendations of 3GPP (3rd Generation Partnership Project,) therefore provide methods whereby the subscriber station estimates a brief channel impulse response hm of the channel from the mth antenna element to the subscriber station and calculates weighting vectors wm to be used to weight the transmitted signal before radiation by the mth antenna element. Corresponding concepts are also dealt with in M. Raitola, A. Hottinen and R. Wichmann, “Transmission diversity in wideband CDMA”, published in Proc. 49th IEEE Vehicular Technology Conf. Spring (VTC '99 Spring), p. 1545–1549, Houston, Tex. 1999.
A serious problem with this method is that the vector of the weighting factors estimated by the subscriber station have to be transmitted to the base station and for this only a small bandwidth of 1 bit per slot is available, according to the recommendations of the 3GPP. The vectors therefore can be transmitted only roughly quantisized. If the channel changes quickly and the weightings have to be updated from one time slot to the other, only two different relative phase positions of the antenna elements can be set. If the channel changes more slowly and, for example, four time slots are available for transmission of the vector, 16 different values of the vector still can be represented.
The known concepts do, however, have their limits. If the number of antenna elements of the base station is greater than two, then the bandwidth required to transmit the vector increases with the number of its components; i.e., the number of antenna elements. As such, a large number of antenna elements would, on the one hand, be desirable to be able to align the transmission beam as accurately as possible but, on the other hand, because of the limited available bandwidth, the weighting vector cannot be updated as often as would be necessary to match the fast fading.
An object of the present invention is, therefore, to provide a method of feedback transmission in a radio communication system of the type described above, and a subscriber station suitable for use in such a system, that enable continuous updating of the weighting vectors used by the base station with a high degree of accuracy matching the speed of change of the weighting vectors while at the same time minimizing the utilization of the uplink bandwidth for updating the weighting vectors.